This invention relates generally to control units for DC-to AC inverters, and more particularly, to a DC-to-AC inverter control unit having a digital addressing memory which contains addressing tables for controlling the electrical switching elements of the DC-to AC inverter as a function of a predetermined amplitude of the DC-to AC inverter output voltage. For each combination of switching elements to be addressed, an addressing table is provided which contains information pertaining to the addressing combination and time information; the periodic scanning of the memory being controlled by a variable frequency oscillator which is connected to a counter.
In a known control unit for a DC-to AC inverter, the control unit is provided with an addressing table which contains the addressing combination and the time information. This known structure further contains an oscillator having a variable frequency output which is connected to the counting input of a counter which controls the periodic scanning of the memory. In this known control unit, one oscillation period of the inverter output voltage is divided into M equal angular steps which are numbered; the counter reading coinciding with the number of the respective angular step. The addressing memory has stored therein in fixed form the numbers of the angular steps at which an inverter valve should be fired or extinguished. An address counter precedes the addressing memory; the information in the memory cells which are addressed by the address counter being conducted to the memory output. The outputs of the counter and the addressing memory are conducted to a comparator which passes a command to the electrical switching elements if the signals at the outputs of the counter and the addressing memory coincide. If such a command is issued, the address counter is advanced simultaneously. The addressing memory contains a plurality of addressing tables, each of which corresponds to a different amplitude of the DC-to AC inverter output voltage. The frequency of the DC-to AC inverter output voltage is given by the oscillator frequency since the predetermined cycle of the addressing is traversed at different speeds which correspond to the oscillator frequency.
In the known control unit, known interrelationships between an input variable, such as voltage amplitude, and an output variable, such as the firing angle or a switching element, can be calculated in advance and stored in the addressing memory in the form of a list. The addressing can also be determined externally using elaborate optimizing methods.
It is a disadvantage of the known control unit, however, that the subdivision into angular steps and the addressing cycle itself are stored permanently in the addressing memory and can therefore not be changed. The resolution provided by the angular steps cannot be chosen to be arbitrarily high, in consideration of the allowable switching frequency of the inverter switching element at high operating frequencies. On the other hand, the fixed size of the steps provides low resolution and is too coarse at low frequency, thereby resulting in a poor approximation of the sinusoidal form. At higher frequencies, it is desirable, such as during motor operation, to convert from the sinusoidal form of the inverter output voltage, to a rectangular form. Such a conversion is not possible with the known arrangement because of the fixed storage of the addressing tables.
It is, therefore, an object of the present invention to develop a control unit of the type mentioned hereinabove wherein the switching frequency within a period, as well as the wave shape of the DC-to-AC inverter output voltage, are variable.